This invention relates to a device for continuously and automatically monitoring the concentration of specified constituents contained in the blood.
More specifically, this invention provides a measuring device not affected either by the change in the quantity of flow passing through the tube which forms the measuring channel or by the drift of the detector at the time of continuously sampling the blood from the blood-vessel of the patient by means of a catheter in order to measure the concentration of a specified constituent of the blood. It further contemplates providing a measuring device in which any mixing ratio of the anticoagulant to the blood can be easily established by using the catheter also having the character of a pump tube when the blood drawing is conducted while supplying the anticoagulant, and which is equipped also with a blood-drawing contrivance of such a kind by the use of which the mixing ratio will remain unchanged even if catheters of "throwaway" type would be replaced one by another in consecutive order.
It must be noted that description will be directed in the following specially to the glucose in the blood, namely, the blood sugar, but it goes without saying that the use of the device according to the invention is not limited only to the blood sugar.
In recent years, a of device that has ability of drawing the blood by a slight amount from a vein of the patient in succession for long hours in order to continuously measure the value of blood sugar has been developed. This device is to be used in the measurement of the pattern of fluctuation in the value of blood sugar while every kind of load test is being performed, or in combination with the bedside monitor of the serious case of diabetes, or joining with the monitor of the blood sugar during and after the operation conducted on the diabetic invalid.
The device of this type is such a one on which the blood is drawn from the living body while being diluted with the anticoagulant by inserting the double-current catheter with an anticoagulant-infusing port and a blood-drawing port into a canula whose pointed head is being held in a vein, and the blood thus drawn is sent to the sensor by a suitable pumping means. In time of correcting the sensor, the double-current catheter is first removed from the canula, and after the correction is completed by the use of a solution of a known concentration, it is connected to the canula again in order to continue the measurement. This system, however, not only is a little too complicated to operate, but also it has a defect that the blood is liable to coagulate within the canula at the time of correcting the sensor.
With the purpose of avoiding such a defect, Japanese Patent Disclosure No. 52-135795 of 1977 U.S. Pat. No. 4,119,406 proposed the provision of a channel-switching means at the channel leading to the sensor.
In the channel-switching means referred-to, which is connected to the continuously flowing sample liquid source and the reference liquid source to be used at the time of correction, the measurement is usually made in succession while letting the sample liquid flow to the sensor section. On the other hand, when the correction of the sensor is to be performed, the channel coming from the reference liquid source is made open in order to let the reference liquid flow into the sensor, while the sample is made to turn its course away so as not to pass through the sensor section but to flow into the drain section. In this way, there is no longer necessity for detaching the catheter even at the time of correcting the sensor, and it is also possible at the same time to avoid the defect that the blood is liable to coagulate.
As for the reference liquid, which is for use in correcting the sensor so that the proper value of concentration may be indicated when the sample secures a definite dilution ratio, it has in practice a tendency to bring about errors in measurement in view of the fact that the dilution ratio may be a little off the established value under the influence of a delicate deformation, for instance, of the channel(tube) if being left intact, so that, in order to hold the tendency mentioned above in check and to detect the actual dilution ratio attributable to the anticoagulant and buffer solution contained in the blood sample which has been withdrawn from the catheter, the prior art referred-to here adopted such a method as comprising the steps of: first picking separately the liquid which is regarded as identical with or equal to the above blood sample, next preparing one sample to be used for the correction by diluting the liquid thus picked at a fixed dilution ratio, then reading the measured value of the sample thus prepared while letting it flow from the channel(tube) for the correction directly to the sensor, and lastly obtaining the actual dilution ratio by comparing the measured value thus obtained wih another measured value having been read when the first sample was allowed to flow from the catheter.
Nevertheless, this prior art (Japanese patent application, No. 52-135795 of 1977) still involves a weak point that must be resolved, as described below, which arises from the fact that the sample source and the reference liquid source are designed in such a manner that they have their own channels.
As a matter of fact, when a single tube is used in succession for long hours, the degree of the change in the state of stream, such as the amount of flow is nonnegligible. For instance, it is not rarely the case that the change in the amount of flow rises over 10% when the tube is in succession for 10 hours. As the thinkable factors of the change in state of stream are pointed out the two become apparent: the change with the passage of time of the tube itself under the pressure of the rollers set on the tube pump, and the other is the contamination due to the adhesion of foreign matters, such as blood-corpuscles, protein, and others contained in the liquid, to the inner wall of the tube. Referring to the former out of the above two factors, since the respective changes with the passage of time of all tubes are different from one another on the basis of the difference in wall-thickness and quality of material and further owing to the subtle unlikeness in construction of the tube pumps used, it may safely be said that it is impossible to predict properly the degrees of their changes. As for the latter factor, on the other hand, it will readily bring about a greater difference between the sample source and the reference liquid source as against the former factor when the separate channel systems are used for each of the both sources because the liquids which flow through the respective channel systems are different in quality.
As a natural consequence, the rate of dilution of the sample and buffer solution, for example, also varies progressively at the time of measurement on the basis of the difference of the way of changing in the state of stream between the channel of the sample source and the channel of the reference liquid source during the continuous measurement, so that the state corrected in accordance with the measured value of the actual dilution ratio at the commencement of the measurement falls gradually into disorder. In the face of such a situation, it is useless even if trying to compensate mutually the respective changes in stream with the passage of the time of both the channel of the same source and the channel of the reference liquid source, because their changes each run independently of other. Further, it is to be noted that it is only on the sensitivity of the sensor that the correction by the reference liquid can be made at any time, while the difference between the changing portion in the channel of the sample source and the changing portion in the channel of the reference liquid source can not be compensated, granting that the mixing ratio of the sample solution to the buffer solution, namely, the dilution ratio is varied, and consequently the measured value obtained in this case comes to include some error.
What is more, although the dilution ratio compensating means using the correcting sample which is regarded as identical with or equal to the above-mentioned sample is effective when the concentration of glucose contained in the sample can be considered unvaried within the time of correction, yet it is difficult in practice to make and use the correcting sample equivalent to the sample under the measurement, because it is a normal thing in fact that the concentration of glucose displays the change with the passage of time during the measurement and yet how it varies can not be foreseen. Suppose the correcting sample is made during the measurement and used, as mentioned above, the making of it not only requires much labor in itself but also the changing portion of the concentration of glucose contained in the sample at the time of correction comes to be exhibited wholly as the error in measurement as it is.
To sum up, there is a possibility of the amount of flow of the time pump changing with the passage of time in every tube of it, so that it is required for reasons of correctness in measurement to compensate these changes one after another. Judging from this point of view, the correcting sample of such a type as in the above-mentioned prior art is said not to be suitable for the purpose of being made one after another during the engagement to be used, and consequently it is impracticable to make the successive compensation by the use of such a kind of a correcting sample.
This invention which has been contrived with the object of eliminating various imperfections in the conventional technique inclusive of the above-mentioned prior art, intends to provide an improved device in which a single channel(tube) is made available in common for both the sample solution and the correcting liquid and on which the blood sugar value can be monitored in succession for long hours with exactitude on the basis of the successive connections of the device itself is conducted. Further, this invention contemplates to provide an improved device on which the sampling can be accurately performed without the dilution ratio showing the scattering caused by the anticoagulant even if catheters would be replaced one by another in the consecutive order as a result of having applied a special shift to the structure of the catheter and also making the catheter have the character of a pump tube.